The present invention relates to wireless communications system and more particularly to multi-user (MU) multiple-input multiple-output (MIMO) wireless communications system.
The present invention considers the problem of designing efficient channel state information (CSI) feedback schemes in order to allow improved multi-user multi-input multi-output resource allocation at a base-station (BS), resulting in increased system spectral efficiency. A cell in which multiple users feedback CSI and the BS performs MU-MIMO resource allocation is depicted in FIG. 1.
Referring to FIG. 1, user terminals 110, e.g. users 1 (111) to K (119), send quantized channel feedbacks 120 to base station 130. At base station 130, DL (downlink) MU-MEMO resource allocation 140 is performed according to quantized channel feedbacks 120 and streams, e.g. user 1 stream 151 to user K stream 159, are subjected to RB (resource block) and/or MCS (modulation and coding scheme) allocation and transmit precoding 160. Signals are transmitted via nT antennas from base station 130 and received by nR antennas, for example, at user 1 (111).
Note that the quality of resource allocation done by the BS depends on the accuracy of each user's CSI report. On the other hand, allowing a very accurate CSI feedback can result in a large signaling overhead. The key challenges that need to be overcome before spectral efficiency gains from MU-MIMO can realized are, for example, as follows:                Improving CSI accuracy without a large signaling overhead, or        Exploiting the enhanced CSI reports at the BS in an efficient manner.        
In order to solve the above problem, others have proposed various solutions, such as increasing CSI feedback overhead; CSI feedback under assumptions on BS scheduling; and complex algorithms for joint scheduling.
CQI (Channel Quality Indicator)/PMI (Precoding Matrix Indicator) reporting enhancements targeting DL MU-MIMO operations on PUSCH 3-1 as well as PUSCH 3-2 were considered by several companies [1]. The proposed enhancement to PUSCH 3-2 comprised enabling sub-band PMI reporting in addition to the sub-band CQI reporting. On the other hand, enhancements to PUSCH 3-1 that were considered suggested that in addition to 3rd Generation Partnership Project (3GPP) Release (Rel-) 8 Mode 3-1 feedback, a user equipment (UE) can be configured via higher layer signalling to report as follows:                A wideband PMI calculated assuming restricted rank equal to one, along with a per subband CQI targeting MU-MIMO operation.        The MU-MIMO CQI is computed assuming the interfering PMIs are orthogonal to the single-user (SU) MIMO rank 1 PMI and for 4 TX, the total number of co-scheduled layers is assumed to be 4 at the time of MU CQI computation [1].        
We propose a broad framework for enhanced CSI reporting by the users in order to obtain an improvement in MU-MIMO performance. We also illustrate mechanisms using which the eNodeB (eNB) can exploit such enhanced CSI feedback. System level simulations show that a simple form of enhanced feedback results in substantial system throughput improvements in homogenous networks and more modest improvements over heterogeneous networks.    [1] Alcatel-Lucent, Alcatel-Lucent Shanghai Bell, AT&T, ETRI, Icera Inc., LG Electronics, Marvell, NEC, New Postcom, Pantech, Qualcomm, RIM, Samsung, Texas Instruments,“Way Forward on CQI/PMI reporting enhancement on PUSCH 3-1 for 2, 4 and 8 TX,” 3GPP TSG RAN WG1 R1-105801 62bis, Xian, China, October 2010.